In the heat of this summer's political wars, Rep. Stanford E. Parris (R-Va.), fighting to retain his congressional seat, sent out a constituent newsletter to the 240,000 households in his district. Printed and mailed at government expense, it listed several "misstatements" about his record and next to them "the truth" according to Parris.

Parris' rival in the race, Herbert Harris, says the newsletter was a direct response to charges he has made against the congressman -- in effect, a piece of campaign literature mailed at taxpayers' expense.

Parris says the letter talked about "the issues of the day. What else would we talk about? We don't send newsletters just to chitchat."

Whoever is right, one thing is certain -- only an incumbent could send a letter to 240,000 voters for free. It's one of the perquisites of office, the powers that make it hard to knock a congressman off the Hill. Indeed, political scientist Norman Ornstein of the American Enterprise Institute has found that 90 percent of incumbent congressmen who ran for office since 1960 have been reelected.

"If you can't get elected as an incumbent," said one longtime congressional aide, "you're either crooked or you're in the middle of a Depression with your party in power. You've got to be in trouble to lose with all the advantages they build in for you."

Though that's an exaggeration, the powers of incumbency are potent, and it's hard to find a Washington-area member who doesn't indulge in their use.

When Social Security was America's hottest topic, Rep. Roy Dyson, a freshman Democrat from Maryland's Eastern Shore, used his office expense fund to produce a low-cost television show on the problems of the elderly for the voters back home. When some folks in Prince George's County were lobbying for a new stamp to commemorate the nation's "first balloon flight," which took place in Bladensburg, their representative, Steny Hoyer, commended them in the Congressional Record and made sure they got copies of his remarks.

In theory it's all done in the name of public service. But members of Congress, like the president, have an ability to command attention that their challengers can only dream of matching, and no one argues that it hurts your reelection chances to get your face on television or your constituents' names in the official record of congressional affairs.

Rep. Michael Barnes (D-Md.), for example, just happened to schedule a press conference to accept a citizen group's nuclear freeze petition on Sept. 15 -- the day after the Maryland primary. The fact that he had just acquired a Republican opponent, Elizabeth Spencer, was probably more than a coincidence.

The most practical advantage of incumbency, observers say, is the congressional privilege to send an almost unlimited amount of official mail by using the congressional "frank" instead of a postage stamp. In the final three months of 1981, the House of Representatives spent more than $11 million to send out franked mail.

Earlier this month, as the preelection cutoff for mass mailings approached, the Longworth and Rayburn office buildings began to look like warehouses. Shoulder-high stacks of newsletters lined the cavernous halls, in a boggling assortment of "Reports to the People" and questionnaires asserting that your congressman "wants to know your views."

Upstairs in the Cannon Building, the staff of the commission on mailing standards was poring over a deluge of last-minute letters, scrutinizing them for any violation of guidelines like the one forbidding a member's name, or the words "I," "me" or "the congressman" from appearing on the average more than eight times per page.

Acknowledging the power of the mails, the House has passed a regulation cutting off mass mailings 60 days before the general election. But the rule has a loophole big enough to drive a mail truck through.

Mass mailings are "deemed to be in compliance" if they arrive at the House Folding Room at least 62 days before the election, with instructions for "immediate dispatch." But with the mountains of mail to be processed, congressional aides say it is anybody's guess just when it all gets out.

A printer delivering one congressman's letters with only hours to spare before the midnight Sept. 1 deadline speculated that many members try to get in as late as they can, so the mail will hit their districts closer to Nov. 2.

Parris, whose final mass mailing went to the folding room on Aug. 31, said it would "be less than candid if I said there wasn't some truth to that."

Indeed, the seesaw battle between Parris and Harris, each of whom has turned the other out of Congress once, illustrates the most ingenious uses of the mails.

During the 1980 campaign, according to a Parris aide, then-congressman Harris "dropped a newsletter on us in October. He must have sent it down the last day." To that, Harris responded demurely, "Any letter we sent out went to the folding room on time. After that we lost control. I don't remember it being an issue."

Another potent power of office is instant access to the people in Washington who get things done, from the bureaucrat who can answer a constituent's question to the president who can grant a congressman's fondest wish.

As a top White House aide from the Carter administration explains, "The most prestigious thing is to let the congressman come in and make a presentation to the president, and then let the press know that the president has agreed to do whatever inane thing the congressman wants for his homestate."

Better yet is to have the president respond to an important constituent concern. Last December, for example, when President Reagan was set to end the Carter administration's policy of charging federal workers fees for previously free parking spaces, he called three local Republicans in for a chat.

When Marjorie Holt of Maryland and Frank Wolf and Parris of Virginia left the Oval Office 15 mintues later, they walked out with the announcement that Reagan was abolishing a program that had infuriated a huge number of their constituents.

Soon, the three members were announcing the decision to the White House press, their names now indelibly linked to the good news.

"Certainly it's helpful to be able to go down to the White House and make your case," said Wolf, who has good reason to know.

Last April, on the morning after Wolf Trap burned, Wolf happened to be having breakfast at the White House. He started lobbying Deputy Chief of Staff Michael Deaver for federal funds to rebuild the performing arts center.

Wolf was named to the committee to rebuild the park, and when the federal grant was announced, the congressman was there at the press conference.

Of course, even the best-laid plans do not always work out completely. When Wolf held a community hearing this month to discuss opening up the Dulles Access Road to area residents, his Democratic opponent, Ira Lechner, had someone in place to sign him up as the first witness. Lechner had his say, and not incidentally, managed to grab some valuable air time on the evening news.

Despite the occasional one-upmanship by a challenger, incumbents generally have the upper hand. Their annual staff allowances of $352,000 and "official expense" allowances of $110,000 on the average can buy an awful lot of constituent service.

Dyson was able to use about $300 from his official expense account to tape his Social Security forum in the fully equipped recording studio available to members. Then he bought $325 worth of air time on the CBS affiliate in Salisbury, the largest city in his district.

The half-hour show aired twice, most recently on Aug. 15 sandwiched between Sunday sports and the CBS network news, according to a station official. "This is basically a public affairs show," said Carol Oldach, sales coordinator for WBOC-TV. "But let's face it, it does him good to be in the public eye."

Oldach said the station did not have to give Dyson's possible Republican opponents equal time because the show was a public affairs presentation and Dyson made no mention of running for office.

Members can also use their official expenses to put together lists of constituents interested in specific subjects. "Just call it the power of the mailing list," said the man running one challenger's campaign.

Each time there's an important development in that area, the congressional computers spew forth "Dear Friend" letters that open with the line, "Knowing of your interest in" the environment, education or whatever is appropropriate.

Congressional action on federal retirees triggers about 1,200 "Dear Friend" letters to Barnes' Maryland constituents. A new twist on Dulles Airport, and Wolf's computers start churning out missives to about 1,000 Northern Virginians. Comes a development on federal workers, and Parris' system whips off as many as 6,000 blue-and-white "Federal Papers."

And if members want to augment their own staffs, the federal agencies are just down the street. When Parris wanted to send a Social Security pamphlet to the 40,000 senior citizens in his district, he borrowed someone from the Social Security Administration to help put it together, an aide explained. It cost about $2,000 to print copies of the handsome blue booklet that recently appeared in doctors' offices and senior citizens' mailboxes emblazoned with the message, "Compliments of Your Congressman Stan Parris."

If consumer brochures are popular back home, then a congressman may draw on the 10,000 offered free each year to each of the members by the Department of Agriculture.

In fiscal 1981, the department spent a little over $1 million to print the publications that cover everything from "Beekeeping for Beginners" to "How to Buy Poultry." Last year, like many of his colleagues, Hoyer made the list available to his constituents and funneled more than 1,100 requests for the books back to the department.

Despite these built-in powers, Congress members are quick to point out that incumbents are not invulnerable. Sometimes they lose, particularly in the politically volatile Washington area.

That, congressional observers say, gives members even more impetus to exercise the powers of office. "All a congressman needs is one example of the incumbent who loses," said political scientist Ornstein, "and they say, 'There but for the grace of God go I.' "